Embodiments of the present invention relate generally to disk drives. Specifically, embodiments of the present invention relate to methods and apparatuses for performing dynamic sampling of a back electromotive force (BEMF) measurement.
In hard disk drives back electromotive force (BEMF) is used as a way to measure the velocity of an actuator arm. It is important to be able to accurately control the velocity of the actuator arm without having to access the positioning information of the read/write head, which is attached to the arm. For Self-Servo-Writing applications, accurate velocity control translates to accurate positioning of the read/write head during repeated motions across the disk surface. Accurate velocity control also prevents mechanical stresses from occurring on the slider and head and improves acoustics and reliability, for example, in ramp Load/Unload operations. Further, accurate velocity control allows for more mechanical design options (i.e., the design of ramps) which may increase the usable area of the disk surface and in turn save manufacturing costs.
In a hard disk drive, a BEMF voltage drop is created across the coil while the coil is moving in a magnetic field. The voltage observed across the coil during normal operation is the combined effect of actuator BEMF (a function of actuator angular velocity) and the impedance of the coil (both resistive and inductive effects) in response to a voice coil motor (“VCM”) command. Changes to the VCM command will (through the impedance of the coil and amplifier dynamics) contribute to voltage drops across the VCM that are not due to induced BEMF.
In conventional disk drive applications, the sample rate for BEMF measurements is limited because one has to wait for the voltage in the coil to reach a steady state after a command change. Various methods have been used to collect BEMF measurements by measuring a voltage across a coil. In order to take the measurement, the VCM driver is disengaged then the coil voltage is measured after a set amount of time has elapsed. To obtain measurements with a constant VCM current a calibrated inductive and/or resistive (“IR”) cancellation is employed. In addition, the VCM current is kept constant for a specified amount of time.
U.S. Pat. Nos. 6,611,118 and 6,864,656 are directed to a magnetic disk drive and voice coil motor drive circuit. The disk drive has a BEMF sensor that detects the BEMF produced in a VCM actuator. The BEMF sensor accepts as input the coil terminal voltage of the voice coil motor and outputs the voltage as a band limited signal and accepts as input the drive current of the voice coil motor and outputs a voltage proportional to the drive current. A computation circuit takes the difference between the two outputs to obtain the BEMF measurement.
The main disadvantage of the above-described methods is time delay. In order to take the BEMF measurement the known systems must wait a discrete time period for transient voltages across the coil due to amplifier/coil dynamics to sufficiently decay. In turn, the sample rate of the BEMF measurement is limited. Further, if the BEMF is sampled at too fast a rate, the accuracy of the measurement will be degraded. This has a direct impact on the responsiveness of the control system for the hard disk.
Accordingly, a method and device is needed to cancel the effects of the coil impedance and amplifier dynamics on the voltage drop across a VCM. Further, a method and device is needed that is capable of sampling BEMF measurements at a rate comparable to the rate at which the VCM command signal changes.